1. Technical Field
This invention relates generally to engines adapted to operate in a homogeneous charge compression ignition (HCCI) combustion mode and more particularly, to an apparatus and method for controlling HCCI combustion in an engine during transient operations.
2. Background Art
Homogeneous charge compression ignition (HCCI) is a mode of combustion in which a pre-mixed fuel/air charge is elevated in temperature during the compression stroke until the thermodynamic conditions in the pre-mixed charge are satisfactory for the initiation of combustion. HCCI combustion occurs when a lean homogeneous mixture of fuel and air begins combustion toward the end of the engine compression stroke. During transient operations, fueling control is very fast because the desired injection mass can be controllably delivered to each cylinder on a cycle-by-cycle basis. However, in a turbocharged engine having an exhaust gas recirculation system, turbo-lag, recirculated exhaust gas transport delays, and the filling-and-emptying dynamics of the combustion chambers, the in-cylinder conditions evolve at a much slower pace, typically occurring over several engine cycles. Even when the air-path responses are enhanced by a closed-loop controller, the in-cylinder conditions stabilize at a much slower pace than the change rate of the fueling. In order to increase engine output torque during on an up-transient change, the fueling rate is primarily determined by the desired BMEP (break mean effective pressure) and engine speed. Thus, mismatches can occur between the actual in-cylinder conditions and the fuel supplied to the cylinder during transient operations. For HCCI combustion, it is not desirable to have significant mismatches between in-cylinder conditions and fueling, because undesirable combustion behavior, such as misfire and knocking can occur.
Several methods have been proposed for transient control during HCCI combustion. For example, United States Patent Publication No. 2006/0196469A1, published Sep. 7, 2006 to Kuo et al. for a Method for Load Transient Control Between Lean and Stoichiometric Combustion Modes of Direct-Injection Engines With Controlled Auto-Ignition Combustion, describes a method for controlling gasoline-fueled direct-injected engines during transient operation in a HCCI mode. The described method synchronizes the change rates of predetermined controlled inputs to the engine fueling rate. A similar publication, United States Patent Application Publication No. 2006/0196468A1, also published Sep. 7, 2006, to Chang et al. for a Speed Transient Control Methods for Direct-Injection Enginess With Controlled Auto-Ignition Combustion, proposes a control method in which controlled inputs are synchronized to changes in the current engine speed and also with any concurrent changes in the engine fueling rate. However, as noted above, although engine fueling rates can be adjusted very quickly, on a cycle-by-cycle basis, the intake air charge composition cannot be adjusted as quickly and therefore fueling and intake air charge mismatches occur. Moreover, in typical engine operation, and particularly in diesel engines, the exhaust gas stream contains a considerable amount of oxygen that was not consumed in the combustion process. Therefore, the recirculated exhaust gas contains oxygen that is added to the intake air introduced into the intake manifold of the chamber. Consequently, even with rapid fueling changes, the oxygen content of the intake manifold air charge is mismatched with the amount of fuel required to meet the increased BMEP demands, and misfire and knocking will occur during the up-transient excursion.
The present invention is directed to overcoming the above-described problems associated with transient operation control of an engine operating in a HCCI combustion mode. It is desired to have an apparatus and method in which the intake manifold air charge is coordinated to track a desired air path signal and thereby adjustably regulate the oxygen content of the intake charge. It is also desirable to have an in-cylinder condition guided fueling function and an apparatus and method whereby excess oxygen in the recirculated exhaust gas is consumed prior to reintroduction into the intake charge to achieve fast and smooth engine response during transient operation.